1. Field of the Invention
The invention relates to an ionization device, more particularly to an electrospray-assisted laser desorption ionization device, which is adapted for use with a mass analyzer and a detector for conducting direct analysis of mass spectrometry on a sample (in particular a biochemical sample), and for obtaining mass spectrometric analysis information on macromolecules such as protein molecules. The present invention also relates to a mass spectrometer utilizing the electrospray-assisted laser desorption ionization device, and a method for mass spectrometry.
2. Description of the Related Art
Laser desorption mass spectrometer performs laser desorption (LD) by utilizing a transmission mechanism that is capable of transmitting laser beams in a vacuum environment. In other words, by irradiating laser beams at the surface of a tissue section, the protein molecules at the site of impact absorb the energy of the laser beams to thereby directly desorb from the surface of the tissue section in the form of ions carrying electric charges. Mass spectrometric analysis is then performed by a mass analyzer. For relevant techniques, please refer to the following article: Tabet, J. C., Cotter, R. J. Anal. Chem. 1984; 56, 1662. It is widely recognized that among the analytes desorbed by the laser beams, the number of neutral analytes far exceeds the number of ionized analytes; that is, ionization efficiency is extremely low. The signal resulted from this extremely low ratio of ionized analytes is too small and is therefore easily interfered by noise signals. At the same time, detection sensitivity and reconstruction ability of the signals are poor such that results of the mass spectrometric analysis is relatively less objective, and is therefore hardly determinative.
Another type of ionization method is electrospray ionization (ESI), which involves extraction of proteins from a tissue section for obtaining a protein solution, followed by a protein analysis conducted by an electrospray ionization mass spectrometer (ESI-MS) 1 including an electrospray ionization device 11 as illustrated in FIG. 1. For relevant technology, please refer to the following article: Yamashita, M., Fenn, J. B. J. Phys. Chem. 1984; 08, 4451.
The electrospray ionization device 11 of the electrospray ionization mass spectrometer 1 performs an electrospray ionization procedure to ionize the proteins in the protein solution. The electrospray ionization device 11 includes a capillary 112 having an open end 111 that opens toward an entrance side 121 of a mass analyzer 12 included in the electrospray ionization mass spectrometer 1. When in use, an electric field, for instance, a 2-5 kV voltage difference, is established between the open end 111 of the capillary 112 and the entrance side 121 of the mass analyzer 12. Subsequently, the protein solution is pushed through the capillary 112 toward the open end 111. The protein solution forms a Taylor cone 2 that is filled with electric charges as it passes through the open end 111 of the capillary 112 due to the combined effect of the electric field present between the open end 111 of the capillary 112 and the entrance side 121 of the mass analyzer 12 and the surface tension of the protein solution at the open end 111. As the electric field force overcomes the surface tension of the protein solution at the open end 111 of the capillary 112, aerosol droplets containing multivalent electric charges and protein molecules are formed, and are pushed into the mass analyzer 12 through the entrance side 121 thereof.
As the charged droplets travel through the air from the open end 111 of the capillary 112 toward the entrance side 121 of the mass analyzer 12, the liquid portion of the charged droplets vaporize such that the charged droplets dwindle in size, causing the multivalent electrons to attach to the protein molecules to form ionized protein molecules with relatively lower m/z values (i.e., the mass-to-charge ratio, where m is the mass of the ionized molecule, and z is the ionic charge/number of elementary charges). Since the molecular weight of a macromolecule, such as a protein molecule, is in the hundreds of thousands, charges attached to each of the macromolecules for forming the ionized molecules needs to be multivalent in order for the m/z value to be low enough so as to be detectable by the mass analyzer 12. Not only does the electrospray ionization method allow macromolecules to be efficiently ionized, but it also overcomes the detection limit imposed by the mass analyzer 12 since a lower m/z value can be obtained. Therefore, protein molecules can be studied using electrospray ionization mass spectrometry.
Although the electrospray ionization mass spectrometer 1 as illustrated in FIG. 1 is capable of conducting mass spectrometric analysis on proteins, the sample used for the analysis can only be in a solution form. Therefore, for a tissue section, mass spectrometry can only be conducted after a series of tedious sample preparation procedures, such as the extraction of proteins and the formation of the protein solution, have been completed. This sample preparation process is time consuming. In addition, detailed spatial analysis of the sample can only be performed if an extremely small voluminal tissue section is sampled and analyzed multiple times using electrospray ionization mass spectrometry.
It can be seen from the above that conducting protein analysis directly on a tissue section using mass spectrometry techniques presents a variety of difficulties and inconveniences. Since spatial analytic information of proteins in organs or tissues is extremely important in medical and biotechnological fields, there exists a great need for a method of mass spectrometry that is capable of conducting rapid, convenient, and accurate protein analysis on a particular portion on an “unprocessed” tissue section (i.e., a tissue section without sample preparation).